The present invention relates to a motion sensor for sensing the velocity of a moving web or belt and, more particular, to a motion sensor which uses a pair of linear position sensors to detect the passage of holes formed in the circumference of a photoreceptor belt to measure the velocity of the belt.
Highlight color and full color printers may use one or more imagers, such as Raster Output Scanners (ROS) or LED print bars, to expose the surface of a photoreceptor belt to form a plurality of superimposed color images. For the highlight color case, two exposures are made on the same exposure frame, or pitch, of the belt. Each exposure is developed with a characteristic color plus black. For a full color image, a single imager, in a multi-pass mode, or a plurality of imagers, in a single pass mode, form successive color images on successive exposure frames with a composite color image being developed and transferred to a copy sheet. Each color image must be registered with previously exposed and developed images with a high degree of accuracy. One source of registration error is velocity variations in the rotational velocity of the photoreceptor belt. A number of factors can contribute to changes in the photoreceptor velocity from a velocity at start of operation such as: variations in the speed of the belt drive motor; runout errors of the drive and tension idler roller; cleaning blade to photoreceptor surface interaction and belt thickness variations. These belt velocity variations cause the spacing between scan lines being formed by the imager on the photoreceptor surface to vary resulting in a visible "banding" effect in the output copies. Also, for a single pass color system, where three or four images are successively formed in a single revolution or pass of the photoreceptor, the leading edge of each exposure frame may vary from the precise registration requirements, It is therefore evident that there exists a need for some mechanism for monitoring the velocity of the belt in the process direction and for correcting for noted changes in the measured velocity.
There are several prior art references which disclose belt sensing and registration correction techniques. The most generally used mechanism is to mount an encoder in contact with a portion of the belt. The encoder provides a series of pulses per revolution, each pulse representative of an incremental distance of belt movement. The encoder outputs are sent to a drive servo control via a feedback loop; the servo control detects changes in the velocity of the belt and adjusts the motor drive of the belt. See, for example, U.S. Pat. No. 4,485,982 for an encoder used in a belt tracking system.
Encoders and related circuitry are expensive components; other less expensive techniques have been developed to produce signals representing the velocity of the belt or web. U.S. Pat. No. 4,837,636 discloses a system in which a belt is formed with a series of holes along one edge. The holes are illuminated and cause a CCD sensor array beneath the belt to be continually illuminated by the light source. Output signals from the sensor array are converted into clock signals which are used for controlling the belt drive motor velocity. U.S. Pat. No. 5,204,620 discloses a plurality of rectangular slits formed along the edge of the belt. The slits are illuminated and flux is incident on a segmented CCD position sensor on the other side of the belt. The sensor light output is sampled and the output during the sampling period is operated upon by a centroid processor to accurately determine the center of movement for each sampled intensity distribution. These prior art devices utilize CCD sensor arrays which, while less expensive than the encoder arrangements, are still a relatively expensive component.
The present invention is directed towards a mechanism for measuring velocity by using relatively inexpensive linear position sensors. In the embodiment which is used to disclose the sensor arrangement, narrow belt slits are formed in a belt in a cross-process direction and a light source is provided which illuminates the full width of the slits as well as the combined lengths of a pair of linear position sensors joined in a staggered arrangement. The light passing through the slits as they advance over the staggered sensor arrays provide output signals over time which represent the photoreceptor velocity. More particularly, the present invention relates to a printer having a photoreceptor belt moving in a process direction, said belt having a series of slits spaced from each other and extending in a row about the circumference of the belt with the length of said slits aligned in a direction generally perpendicular to said process direction, the combination of:
a sensor system for measuring the velocity of said belt, said sensor system including a pair of linear position sensors adjacent one surface of the belt, said sensors constructed in a staggered configuration to provide a continuous length longer than the length of either sensor while shorter than the slit spacing, said sensors aligned so that their lengths are generally parallel to the process direction, and an illumination source positioned on the other side of the belt and adapted to provide a band of illumination which illuminates the length of said slits and the combined length of said sensors, PA1 drive means for rotating said belt so as to move said slits across the combined length of said sensors to allow a continuous band of illumination from said light source to fall on said sensors, PA1 switching means for switching the operational state of said sensors as a function of the position of said slit and the output voltage of said sensors so that output signals are alternately generated by each sensor, and PA1 circuit means for differentiating said sensor output signals to produce a signal representing the velocity of the belt.